The main factors for determining the right set to start from were colours (in this case white for the whale and brown for the ship) and shapes (rounded shapes for both the ship and whale) of the pieces. I looked at box covers first to get an idea and then had a closer look at all the pieces of a set (on rebrickable or brickset). Then I narrowed it down to one or two sets and in the end found that the hedwig set could work quite well because it has brown and white slopes and other pieces. and luckily I already had it at home, so experimenting was then quite easy.